The purpose of this proposal is to better understand the effects of two common treatments for end stage ankle arthritis (ESAA). Until fairly recently, the gold standard for treatment ESAA has been an ankle arthrodesis (i.e., a fusion). However, recent improvements in the long-term viability of ankle arthroplasty (i.e., joint replacement) have increased the treatment options available. Many patients and physicians are now considering ankle arthroplasty, whereas a decade ago, most would only consider arthrodesis. Although arthrodesis can remove the primary symptom of ESAA (i.e., pain), the patient is left with a fused ankle, which results in the distal foot joints (subtalar, midtarsal and tarsometatarsal) being stretched into increased dorsiflexion during the push off phase of the gait cycle. Subsequently, a common secondary complication of arthrodesis is the development of arthritis in the distal joints of the ipsilateral foot. It is thought that joint replacement, which allows for more normal ankle motion, will not lea to aberrant dorsiflexory moments on the distal foot joints. While certainly plausible, a detailed comparison of the motion of the bones of the foot in patients with an arthrodesis vs. an arthroplasty has not been conducted to date. Moreover, while it is known that both treatments for ESAA are susceptible to misalignments of the talus relative to the tibia, the specific effects f poor alignment on the distal joints are not well understood. Finally, the last issue this proposal will address is that while there is general agreement on the ideal patients for both procedures (arthrodesis patients tend to be young and more active, while arthroplasty patients are often older and less active), this clinical wisdom is not absolute. Moreover, there are many subjects who fall between the ideals and it is not known which treatment they will benefit from most. As such, we propose the following: Specific Aim 1: To determine the foot joint kinematics of subjects treated for ESAA with either an arthrodesis or arthroplasty. We will employ a multi-segment foot model that our group has developed to study 60 subjects (n=30 for each group) pre-surgery and then at 12 and 24 months follow up. Specific Aim 2: To determine the effect of joint alignment on foot joint kinematics with cadaveric feet simulating treatment for ESAA with either an arthrodesis or arthroplasty. Using the Robotic Gait Simulator (RGS), a cadaveric gait simulator that our group has developed, we will systematically vary the alignment and quantify the effect on foot joint kinematics for ankle fusion and total ankle joint replacement in 30 cadavers each (60 total). In the neutral position, we will also be able to directly compare arthrodesis to arthroplasty by quantifying the motion of individual bones in the foot (not possible with living subjects). The secondary aims of this study are to compare the following parameters pre- and post-operatively between ankle arthrodesis and arthroplasty patients: physical function, ankle specific function, pain, general health, overall patient satisfaction, step counts and full body motion analysis, as well as post-surgical complication rates. We aim to quantify differences between the treatment groups and to identify intrinsic and extrinsic prognostic factors that are predictive of improved performance. The results of this research project will directly benefit the portion of the general population that experience ankle OA, which has been estimated as 6-10%. Moreover, an increased incidence of ankle OA has been suggested. Osteoarthritis often limits overall mobility and may not only restrict a patient's activities of daly life, but also negatively impact their social and psychological health status. Therefore, it is imperative to address ankle OA conditions with the goal of designing more effective treatment options and interventions to slow the disease progression.